A loop antenna is a radio frequency antenna consisting of a loop, or several loops, of wire, tubing or other electrical conductor with ends of the loop connected to a feed line. Conventional loop antennas are balanced and are conventionally fed with balanced feed lines to avoid balance mismatches between the antennas and the feed lines, or with unbalanced feed lines via baluns. A loop antenna's resonant frequency is determined by circumference of the loop. A loop antenna resonates at a frequency whose wavelength (λ) equals the circumference of the loop and at other odd multiples of that frequency, taking into account velocity factor of the loop element conductor.
Although most loop antennas are circular, distorting the loop into a different closed shape generally does not greatly alter its characteristics. For instance, a quad antenna consists of a resonant loop, and usually additional parasitic elements, in square shapes, where each leg of the square is about one-quarter (¼) wavelength long. In general, gain of a loop antenna is directly proportional to area enclosed by the loop. Thus, other things being equal, a quad antenna exhibits slightly less gain than a circular loop antenna. In many ways, loop antennas can be viewed as deformed folded dipole antennas. For example, loop antennas have electrical characteristics, such as high radiation efficiency, similar to folded dipole antennas. For a given antenna operated in a linear medium, the antenna's transmit and receive characteristics, such as impedance, radiation pattern and sensitivity, are identical.
Absent a ground plane, a loop antenna radiates, or is sensitive, in directions normal to a plane of the loop, thus in two opposite directions. Further directivity can be obtained by increasing the loop circumference to three or five wavelengths. However, it is more common to increase gain by adding a ground plane spaced apart from a driven loop, using an array of driven loops or a Yagi configuration that includes parasitic loop elements. However, all these methods significantly increase overall size of the antenna.
Polarization of a loop antenna is not obvious by looking at the loop itself. The polarization depends on location of the antenna's feed point. If a vertically oriented loop is fed at its bottom, the antenna is horizontally polarized. However, feeding such a loop from a side makes the antenna vertically polarized.
A ground plane may be used to increase directivity, and therefore gain, of a loop antenna by preventing radiation or reception in one direction normal to the plane of the loop. However, loop, dipole and patch antennas dramatically loose gain when driven elements are placed too close to ground planes, which reduces the capability of a system to effetely receive weak signals or efficiently transmit signals. Prior art loop antennas require a significant, i.e., at least about 1/10 wavelength, space between their driven elements and their ground planes to achieve sizable gain. Furthermore, as the distance between a driven loop and its ground plane is reduced, impedance of the loop antenna decreases, such as to about 5-10 ohms, making it difficult or impossible to match a feed line to the antenna.
Gain reductions due to close-spacing of ground planes in loop antennas may be compensated by increasing power used to drive the antennas, or increasing power supplied to receive-signal amplifiers, or by increasing the diameters of the loops. Neither option is particularly attractive, especially in compact, low power consumption systems.